Determination of methionine and selenomethionine in yeast by species-specific isotope dilution GC/MS

A method for the simultaneous determination of methionine (Met) and selenomethionine (SeMet) in yeast using species-specific isotope dilution (ID) gas chromatography/mass spectrometry (GC/MS) is described. Samples were digested by refluxing for 16 h with 4 M methanesulfonic acid. Analytes were derivatized with methyl chloroformate and extracted into chloroform for GC/MS analysis. In addition to use of commercially available C-13- enriched Met and SeMet spikes for species specific ID analysis, a Se-74-enriched SeMet spike was also available for comparison of results. In selective ion monitoring mode, the intensities of ions at m/z 221, 222, 269, 270, and 263 were used to calculate the 221/222, 269/270, and 269/263 ion ratios for quantification of Met and SeMet. Concentrations of 5959 +/- 33 and 3404 +/- 12 mug g(-1) (one standard deviation, n = 6) with relative standard deviations of 0.55 and 0.36% for Met and SeMet, respectively, were obtained using C-13-enriched spikes. A concentration of 3417 +/- 8 mug g(-1) (one standard deviation, n = 6) was obtained using the Se-74-enriched SeMet spike. The concentration of SeMet measured in the yeast is equivalent to 66.43 +/- 0.24% of total Se and 30.31 +/- 0.11% of total Met is in the form of SeMet. Method detection limits (three times the standard deviation) of 3.4 and 1.0 mug g(-1) were estimated for Met and SeMet, respectively, based on a 0.25-g subsample of yeast with 1 mL of extract used for derivatization. A similar concentration of 5930 +/- 29 mug g(-1) (one standard deviation, n = 4) for Met and a lower concentration of 2787 +/- 49 mug g(-1) (one standard deviation, n = 4) for SeMet were obtained for this yeast sample using species-specific ID analysis based on GC/MS with C-13-enriched Met and SeMet spikes when a 2-h open microwave digestion approach using 8 M methanesulfonic acid was used.